You can use the textbox on the bottom to add a general reply to the conversation.
The "Reply" button under a person takes their entire reply and immediately quotes it in the textbox for you to respond to.
The "Quote" button saves that person's reply in a quotes bank. You can then paste all those saved quotes by clicking on the "Insert quotes..." button that pops up under your text box. Bonus: you can save a reply from one thread, and quote it in another with this feature (never tried it).
You can imagine that can get bulky when replying to everyone's full responses. What you can do instead is highlight the specific passages you're responding to. That'll bring up a pop up "Quote | Reply" which only takes your highlights.